Semiconductor memory devices that do not require power to maintain data stored therein are called non-volatile memory devices. Non-volatile memory devices may be widely used in mobile communication systems and/or memory cards due to such data storing capabilities.
Non-volatile memory devices having a Silicon-Oxide-Nitride-Oxide-Silicon (SONOS) structure have recently been developed. Non-volatile memory devices having a SONOS structure may offer many advantages over conventional devices, such as reduced manufacturing complexity and flexibility in forming peripheral and/or logic regions in integrated circuit devices.
FIGS. 1 through 3 are cross-sectional views illustrating conventional SONOS-type non-volatile memory devices. Referring to FIG. 1, a first SONOS non-volatile memory device 1 includes an oxide-nitride-oxide (ONO) film 14 between source/drain regions 12 in a silicon substrate 10. The ONO film includes a tunnel oxide film 15, a nitride film 16, and a control oxide film 17. A control gate 18 is formed on the control oxide film 17. The nitride film 16 may function as a charge trap layer to trap charges which may penetrate through the tunnel oxide film 15. The control oxide film 17 may impede penetration of charges stored in the nitride film 16 into the control gate 18.
Still referring to FIG. 1, when a voltage is applied to the control gate 18, charges (such as electrons and/or holes) may migrate toward the tunnel oxide film 15 between the source/drain regions 12. More specifically, when a positive voltage is applied to the control gate 18, electrons may migrate from the semiconductor substrate 10 and become trapped in a first region 20 of the nitride film 16. On the other hand, when a negative voltage is applied to the control gate 18, holes may migrate from the semiconductor substrate 10 and become trapped in the first region 20. However, in either case, charges may accumulate around the drain, which may be biased at a higher voltage than the source. In particular, when the drain is located below a right side of the nitride film 16, as shown in FIG. 1, charges may accumulate in the first region 20 near the drain.
The charges accumulated in the first region 20 can alter the threshold voltage Vth of the non-volatile memory device. The first SONOS device 1 may have a relatively high initial threshold voltage and program current since the ONO film 14 is formed along the entire channel region.
Referring to FIG. 2, a second SONOS device 2 includes an ONO film 34 between source/drain regions 32 in a silicon substrate 30. The ONO film 34 includes a tunnel oxide film 35, a nitride film 36, and a control oxide film 37. A control gate 38 is formed on the control oxide film 37. The nitride film 36 includes two separate portions with a dielectric film 38 therebetween. The second SONOS device 2 including the separated nitride film 36 is called a localized SONOS device. As such, the localized SONOS device is a 2-bit non-volatile memory device that can store 2 bits of information (one bit in each portion of the separated nitride film) by including two ONO layers under a single gate electrode.
Localized SONOS devices may have reduced program current and increased reading speed, since the trapped charges may be locally confined in each portion of the separated nitride film 36. However, localized SONOS devices may still have a relatively high initial threshold voltage, since the tunnel oxide film 35, the dielectric film 38, and the control oxide film 37 are formed across the entire channel region.
Referring to FIG. 3, a third SONOS device 3 includes an ONO film 54 between source/drain regions 52 in a silicon substrate 50. The ONO film 54 includes a tunnel oxide film 55, a nitride film 56, and a control oxide film 57. A control gate 60 is formed on the control oxide film 57. The ONO film 54 is divided into two portions by a separation oxide film 58. The third SONOS device 3 may have a reduced threshold voltage, since the control gate 60 on the separation oxide film 58 is disposed on a greater portion of the channel region, which may allow for greater control thereof.
However, as semiconductor memory devices are continually scaled-down, conventional 2-bit non-volatile memory devices may not be easily reduced in size.